1. Field of the Invention
The present invention relates to a liquid crystal display panel, and more particularly to an active matrix type liquid crystal display panel provided with an alignment checking mark for checking alignment of a first polarizer with respect to its corresponding substrate in the liquid crystal display panel.
2. Description of the Related Art
In general, the liquid crystal display panel is constructed of: a glass substrate (hereinafter referred to as the TFT substrate) provided with a plurality of thin film transistors (hereinafter referred to as the TFTs); another glass substrate (hereinafter referred to as the CF substrate) provided with a color filter (hereinafter referred to as the CF substrate); a liquid crystal layer sandwiched between the TFT substrate and the CF substrate; and, a pair of polarizers each of which is applied to each of the TFT substrate and the CF substrate. In fabricating such a liquid crystal display panel, it is necessary for each of the polarizers to be precisely aligned in position with each of the TFT substrate and the CF substrate. Further, in order to keep constant in production quality the liquid crystal display panel, it is necessary to check alignment of the polarizer after completion of its application to each of the TFT substrate and the CF substrate. Further, in order to improve the liquid crystal display panel both in production quality and in productivity, it is important to check alignment of the polarizer precisely and in a speedy manner.
Heretofore, checking alignment of the polarizer and the CF substrate has been performed with reference to a side edge of the CF substrate and its corresponding side edge of the polarizer. In other words, by using the polarizer which is similar to the CF substrate in shape but slightly smaller in size than the CF substrate (for example, by the amount of from 1 to 0.5 mm in each side), a gap in each side between the CF substrate and the corresponding polarizer is visually checked. In the case that the CF plate and the polarizer are precisely aligned with each other, the same gap is observed in every side. Consequently, with respect to the CF substrate, it is possible to visually check alignment of the polarizer with practically allowable accuracy.
In contrast with the above, it is not possible to perform an alignment operation of the polarizer with respect to the TFT substrate in the same manner as that described above, because: in general, the TFT substrate is provided with a terminal region for mounting therein a drive IC circuit (i.e., Integrated Circuit serving as a drive circuit), and therefore larger in size than the CF substrate in plan view by the amount of the corresponding area of such terminal region. As for a display area of the liquid crystal display panel, there is substantially no difference between the TFT substrate and the CF substrate. As a result, the terminal region of the TFT substrate is placed off a side edge line of the CF substrate in plan view. Further, in order to avoid interface between the liquid crystal display panel and a mechanism element of a module in which the liquid crystal display panel is mounted, the polarizer of the liquid crystal display panel is restricted in plan view to a plane size larger than the display area of the liquid crystal display panel by the amount of from approximately 1 to approximately 2 mm in each of width and height. In this case, even when the alignment operation of the polarizer and the TFT substrate is precisely performed, a gap between the side edge of the TFT substrate and the corresponding side edge of the polarizer varies in each of side edges of the TFT substrate (for example, within a range of from 1 to 5 mm). Consequently, as for the TFT substrate, it is difficult to visually check alignment of the polarizer and the TFT substrate in the same manner as that used in the case of the CF substrate.
On the other hand, Japanese Patent Laid-Open No. Hei 5-216021 discloses a conventional liquid crystal display panel which is capable of checking alignment of a substrate and a polarizer with the use of an alignment checking mark.
FIG. 7 shows an exploded perspective view of the conventional liquid crystal display panel disclosed in this Japanese Patent Laid-Open No. Hei 5-216021 (hereinafter referred to the prior art 1).
As is clear from FIG. 7, this conventional liquid crystal display panel is constructed of: a glass substrate (hereinafter referred to as the TFT substrate) 101 provided with a plurality of thin film transistors (i.e., TFT not shown); another glass substrate (hereinafter referred to as the CF substrate) provided with a color layer (i.e., CF layer) 109; a liquid crystal layer sandwiched between these substrates 101 and 109; and, a pair of polarizers 102, 107.
The plurality of the TFTs are arranged in a matrix on a surface 101a of the TFT substrate 101, in which a plurality of pixel electrodes (not shown) are electrically connected with these TFTs. The plurality of the pixel electrodes and the TFTs are combined to form a pixel group region 108 shown in gray in FIG. 7. Further formed on the surface 101a of the TFT substrate 101 are: a plurality of scanning lines (not shown) for feeding a selection signal to each of the TFTs; and, a plurality of signal lines (not shown) for feeding a video signal to each of the TFTs.
In the surface 101a of the TFT substrate 101, formed outside the pixel group region 108 are three common electrode wiring layers 104a, 104b and 104c. As is clear from FIG. 7, each of the common electrode wiring layers 104a, 104b assumes an inverted L-shaped form in plan view, while the remaining common electrode wiring layer 104c assumes a U-shaped form in plan view. A predetermined voltage is supplied from an external source to each of these common electrode wiring layers 104a, 104b and 104c. A transfer electrode 110 is formed in each of these common electrode wiring layers 104a, 104b and 104c and electrically connected with these wiring layers 104a, 104b and 104c.
The polarizer 102 is aligned with and mounted in its mounting area 130 (shown in dotted lines in FIG. 7) which is defined in a rear surface 101b of the TFT substrate 101. The polarizer 102 is larger in size than the pixel group region 108 in plan view to cover the entire pixel group region 108 from the rear surface (101b) side of the TFT substrate 101.
Formed in a rear surface 106b of the CF substrate 106 is a color filter (i.e., CF) layer 109, which comprises: respective pigment dots or strips of R (i.e., red), G (i.e., green), and B (i.e., Blue) colors each arranged in a predetermined pattern; and, a black matrix (not shown) arranged so as to fill a space between these pigment dots or strips of R, G, and B colors, the black matrix being made of light shielding metal or black resins. The CF layer 109 defines a display region 111 in the above conventional liquid crystal display panel.
Integrally formed with the black matrix (not shown) around the outside of the CF layer 109 is a belt-like light shielding layer 105. Formed further outside the light shielding layer 105 are eight pairs of alignment checking marks 103, which are made of the same material as that of the light shielding layer 105 and disposed in four corners of the CF substrate 106.
Formed in surfaces of both the CF layer 109 and the light shielding layer 105 are common electrodes (not shown), each of which is made of transparent and electrically conductive material to cover the entire area of the rear surface 106b of the CF substrate including the alignment checking marks 103. The common electrodes (not shown) are electrically connected with the transfer electrodes 110. A predetermined voltage is supplied from each of the common electrode wiring layers 104a, 104b and 104c to the corresponding one of the common electrodes (not shown) through the corresponding one of the transfer electrodes 110.
On the other hand, the other polarizer 107 is mounted in its mounting area 131 which is defined in a front surface 106a of the CF substrate 106. This polarizer 107 is larger in size than the display region 111 in plan view to cover the entire display region 111 from the front surface (106a) side of the CF substrate 106.
In the conventional liquid crystal display panel shown in FIG. 7, by comparing the side edges of each of the polarizers 102 and 107 with the alignment checking marks 103 of the CF substrate 106, it is possible to check alignment of each of the polarizers 102 and 107 with respect to the CF substrate 106.
Incidentally, it is also possible to provide the alignment checking marks 103 in the TFT substrate 101. In this case, it is possible to check alignment of each of the polarizers 102 and 107 with respect to the TFT substrate in the same manner as that employed in the case of the CF substrate 106.
In addition to the above conventional liquid crystal display panel, other conventional liquid crystal display panels are also disclosed in Japanese Patent Laid-Open Nos.: Hei6-82772 (hereinafter referred to the prior art 2); Sho59-9638 (hereinafter referred to the prior art 3); and, Sho62-287222 (hereinafter referred to the prior art 4).
In the conventional liquid crystal display panel disclosed in the prior art 2, there is employed laser radiation to evaporate a part of the light shielding layer, so that the alignment checking mark is formed in the part of the light shielding layer of the CF substrate, which mark makes it possible to precisely align the TFT substrate with respect to the CF substrate in their stacking operation.
Further, in this conventional liquid crystal display panel disclosed in the prior art 2, alignment in the above-mentioned stacking operation is improved in precision by stacking the TFT substrate and the CF substrate with reference to both: a part of the signal lines formed in the TFT substrate; and, the alignment checking mark formed in the light shielding layer.
On the other hand, in another conventional liquid crystal display panel disclosed in the prior art 3, when a sealing compound is printed on the liquid crystal display element, a positioning mark is also printed on the same liquid crystal display element. Further, when a fixed display portion is printed on the polarizer, the positioning mark is also printed on the same polarizer. Thereafter, both the liquid crystal display element and the polarizer are positioned, stacked and bonded to each other with reference to their positioning marks.
As described above, in the conventional liquid crystal display panel disclosed in the prior art 3, such a separate positioning mark is formed in each of the liquid crystal display element and the polarizer and used as a reference mark in the positioning, stacking and the bonding operation of the liquid crystal display element and the polarizer, so that alignment of the polarizer is improved in precision with respect to the liquid crystal display element.
On the other hand, in further another conventional liquid crystal display panel disclosed in the prior art 4, when a patterning process of a transparent electrode is conducted, a positioning mark of a liquid crystal cell is also formed. Further, the positioning mark is also formed in the polarizer. After that, the liquid crystal cell and the polarizer are positioned and bonded to each other with reference to their positioning marks which are aligned with each other.
As described above, in the further another conventional liquid crystal display panel disclosed in the prior art 4, such a separate positioning mark is formed in each of the liquid crystal cell and the polarizer and used as a reference mark in their positioning and bonding operations to improve alignment of the polarizer in precision with respect to the liquid crystal cell.
In recent years, downsizing and reduction in weight of a liquid crystal display panel used in a notebook type personal computers and like instruments have been enhanced on purpose to improve these instruments in portability. In order to downsize and reduce in weight the liquid crystal display panel of the above type, it is necessary to reduce in size an outside peripheral region (hereinafter referred to as the panel frame region) disposed adjacent to the display region of the liquid crystal display panel. Due to the above necessity, a gap between an outer peripheral edge portion of the CF substrate and that of the display region formed on the same CF substrate is reduced so that the light shielding layer is formed in the outer peripheral region of the CF substrate or in the vicinity of such outer peripheral region.
In the conventional liquid crystal display panel shown in FIG. 7, the alignment checking marks 103 are formed outside the light shielding layer 105. Due to this, when the gap between the outer peripheral edge portion of the CF substrate and that of the display region formed on the same CF substrate is reduced, it is impossible to form the alignment checking marks 103, which makes it impossible to check alignment of the polarizer 102, and therefore impossible to find poor alignment of the polarizer 102 in its alignment checking operation after completion of its mounting operation, which causes a decrease in production yield and further makes it impossible to downsize the liquid crystal display panel of the above type. This is true in the case of the alignment checking mark formed in the TFT substrate.
In the conventional liquid crystal display panel disclosed in the prior art 2, a part of the light shielding layer is evaporated (i.e., removed). Consequently, in the thus partially-evaporated light shielding layer, there is produced a region permitting the light to pass therethrough, which causes leakage of light issued from a backlighting source of the liquid crystal display panel, and therefore impairs the quality of display in the liquid crystal display panel.
On the other hand, in the conventional liquid crystal display panel disclosed in each of the prior arts 3 and 4, since the positioning mark is made of the sealing material or material of the transparent electrode, such a positioning mark is hard to see in checking alignment of the polarizer in its mounting operation, which impairs in efficiency the alignment checking operation of the polarizer after completion of its mounting operation.